I agree that you should try to find time to have lunch or dinner with them and if you have some give time in your schedule somewhere then maybe that will work...I also agree that if your mom would like to hang out and visit, then you guys can meet up later in the park...
Mom ALWAYS knows best anyway, lol 
....... But what i would do is call disney and see if you change it to how every many people and send an e-mail out to both familys inviting who ever wanted to come to come to dinner... just polietly tell them that you will be in town and want to see everyone but don't want to leave the resort for blh blh (some nice good reason trafic, didn't rent a car) and would like everyone to meet you there..... And add in there that if anyone would like to meet up for dinner or to do the parks that you would love it and just let you knowto make plans, but if not the family is planning a trip back out to so-cal real soon, and hope you can all meet up next time.
I like the idea of "Hey, we'll be in town, if you can make it to dinner at RF Cafe (or wherever) 5 p.m. Saturday we'd love to see you... No? Can't make it? Oh, too bad, maybe next time."
,but not set in stone.This is not your case,you fortunately just have a small meet desired.The hard part is we do know and see the family from so-cal occasionaly.Now we set up a meet for a dinner or lunch at RF, come out of the park and then go back in.It has worked well as a good compromise.Don't get me wrong I love my in-laws.I think I was over blesssed with them.Hence why a son-in-law would invite them on 3 DL trips in one year.